1. Field of the Invention
The invention relates to valve drives for charge-cycling valves of internal combustion engines.
2. The Prior Art
A valve drive of this type, for outlet valves of four-stroke internal combustion engines is described in French Patent No. FR 976.076, in which an outlet valve held in the closed position by a spring force is optionally activated by a first or a second cam, with different lifts of a camshaft. The two cams form an axially displaceable unit that is mounted on the camshaft so as to rotate with it, but to be displaceable. This unit can be displaced into two different positions by a switching fork.
The first cam is adapted to a charge-cycling process to be controlled, with its elevation curve and angle position relative to the crankshaft, and the second cam is active in a different stroke region than the first cam.
In the first position, the first cam is in engagement with the tappet of the valve; in the second position, both the first and the second cam are in engagement with the tappet. The first cam constantly controls the usual opening of the outlet valve after expansion and during expulsion of the combustion gases. The second cam, which can be alternatively placed into a position that is ineffective or effective for the outlet valve, opens the outlet valve, in its effective position, in addition to its usual opening during the intake and/or compression stroke. In this way, the exhaust gas also goes into the cylinder, in addition to the charge that was drawn in, in the case of intake throttling with a low fill volume of the cylinder, so that a greater compression is achieved.
A valve drive for outlet valves of four-stroke internal combustion engines is described in Japanese Patent No. JP 03-202 603, with an outlet cam that has a second elevation and that can be lowered. In its active, outermost position, this elevation opens the outlet valve in addition to its usual opening during the intake and/or compression stroke. In the lowered position, the second elevation is below or at the position of the cam basic circle, and is therefore ineffective.
It is disadvantageous in these embodiments that due to the alternatively effective or ineffective switching of the second cam, the exhaust gas to be introduced in addition to the charge that is drawn in during the intake and/or compression stroke cannot be precisely metered.
A variable valve drive is described in German Patent No. DE 101 56 309 A1, in which a cam with only one elevation is in engagement with a cup tappet, and an additional hydraulic activation device is disposed in the cup tappet. With this activation device, which is supplied and controlled by an additional pressure supply unit, additional opening outside the region of engagement of the cam elevation is possible and an enlargement of the valve opening beyond the opening process of the valve by the lift of the cam can be achieved.
This variable valve drive for outlet valves is used for implementing an exhaust gas feed-back by an additional, multiple opening of the outlet valves outside of the stroke for expulsion of exhaust gas during the intake and/or compression stroke.
German Patent No. DE 44 24 802 C1 describes a process in which an inlet valve is opened during the stroke for expulsion of exhaust gas, in order to bring about an exhaust gas feed-back from the cylinder into the intake system. The inlet valve is activated by a cam having different elevations. For variable activation of the valve for the additional opening process, independent of the lift of the related cam, the hydraulic cushion of the hydraulic valve place adjustment is utilized.
It is a disadvantage of the two valve drives described above that in order to control the additional opening of the charge-cycling valves, a separate pressure system with a control device synchronized with the crankshaft is required.
A method for operating internal combustion engines with variable gas change control times is described in German Patent No. DE 199 05 364 C1. For a direct feed-back of exhaust gas during the intake and compression stroke, the opening time of the outlet valve extends from the end of the expansion stroke over the expulsion stroke to half of the intake stroke, and greater overlap of the valve opening of the outlet and inlet valve occurs.
To the extent that charge-cycling valves are discussed below, these can be both inlet valves and outlet valves.